1. Field of the Invention
This invention relates generally to the formation and control of individual micron size and submicron size cavitation bubbles for use in nanofabrication operations. More particularly, embodiments of the invention teach methods and apparatus for control of a re-entrant micro-jet formed upon collapse of an individual or array of cavitation bubbles and directing the impact of the micro-jet toward a work surface or other objects with a high degree of precision.
2. Description of the Related Art
In general, the production of cavitation has been a phenomena many have tried to avoid. Cavitation in a liquid is the formation, growth, and collapse of gaseous and vapor bubbles due to the reduction of pressure below the vapor pressure of the liquid at the working temperature. Pump impellers, boat props, and similar applications experience cavitation which can produce rapid damage and erosion of surfaces. It has been well known for many years that ultrasonic cleaning devices, which function by the creation of cavitation bubbles, can produce significant surface damage to even the hardest of materials. Studies by a number of authors have revealed that one significant element in producing the damage caused by cavitation occurs when a cavitation bubble collapses in the vicinity of a surface, launching what is called a re-entrant micro-jet toward the surface. This liquid jet can produce velocities as high as 1500 m/s, and is capable of damaging the hardest materials known.
Recently, a number of applications have been developed utilizing the formation of cavitation bubbles through the use of laser light or electrical discharge. Esch et al. (U.S. Pat. No. 6,139,543) and Herbert et al. (U.S. Pat. No. 6,210,400) disclose the use of laser light introduced into a catheter device for the purpose of creating cavitation bubbles, whose expansion and collapse are utilized to pump fluids in and out of the catheter. Hammer et al. (U.S. Pat. No. 5,738,676) discloses a laser surgical probe with a special lens designed to produce the cavitation bubbles further from the end of the fiber optics, to reduce the damage formed (presumably by the reentrant micro-jets launching into the lens on the end of the cable). Such damage was also reported by Rol et al. in “Q Switched Pulses and Optical Breakdown Generation Through Optical Fibers”, Laser and Light in Ophthalmology, Vol. 3, No. 3, 1990. Palanker (U.S. Pat. No. 6,135,998) describes a method for performing electrosurgery using sub-microsecond, high power electrical pulses applied to an electrosurgical probe interface. The tool described by Palanker provides a cutting force by both the plasma generated by the electrical arc and shock waves produced by collapsing cavitation bubbles.
In each of the prior art references cited above, there has been no attempt to control the direction and impact of the powerful micro-jets formed upon the collapse of the cavitation bubbles created when highly focused energy is introduced into a liquid. Without such control, concern of collateral damage cannot be avoided, especially when such tools are used in the human body in a medical application.
Recently as well, there has been a significant interest generated in the field of nanotechnology, for methods needed to fabricate micron and submicron devices and nanomachines. There are very few fabrication tools available that can cut, drill, peen, deform, or otherwise modify features of a surface on a submicron to nanometer scale. Much of the technology developed by the semiconductor industry requires the fabrication of structures utilizing photolithographic processing. This technology is not as flexible as may be required, and will have certain difficulties when applied to biological nanotechnology systems. Advancing the state of the art required by nanotechnology applications will require fabrication technologies operating at least 1 to 2 orders of magnitude below that capable in the semiconductor process arena.
The invention as described in the above referenced provisional application provides a method for the controlled formation of individual cavitation bubbles comprising immersing a mask including at least one aperture within a liquid, immersing a work piece having a work surface in the liquid proximate to the mask, generating a cavitation bubble proximate to the aperture such that the mask is located between the cavitation bubble and the work piece. A re-entrant micro-jet formed during the collapse of the cavitation bubble is directed through the aperture to the work surface. An apparatus for the controlled formation of cavitation bubbles as described in the above referenced provisional application includes a mask having at least one aperture, immersed in a liquid, and an energy source having an energy flow in the liquid sufficient to create at least one cavitation bubble. The energy flow creates the cavitation bubble proximate to the aperture and the collapse of the cavitation bubble creates a re-entrant micro-jet directed through the aperture to a work surface. While this technique is very useful for processing surfaces that can be located conveniently in the vicinity of a fixed orifice, there are many other situations where one may wish dynamic, three dimensional control of the direction of the re-entrant micro-jet. These situations may include eye surgery, for example, where placing an orifice structure inside the eye may not be practical.
The prior state of the art therefore has yet to provide a fabrication technology capable of operating in the nanometer region by harnessing the powerful phenomena of the re-entrant micro-jet formed during the collapse of a precisely located cavitation bubble. What is further needed is a method and apparatus to precisely control the direction and location of the re-entrant micro jet without the encumbrance of physical structure such an orifice between the work surface and the cavitation bubble.